The present invention relates to a fixing device for a printer, facsimile apparatus, copier or similar electrophotographic image forming apparatus and more particularly to an induction heating type fixing device using electromagnetic induction. Further, the present invention relates to an induction coil for use in the induction heating type fixing device.
Generally, an image forming apparatus includes a fixing device for fixing a toner image on a paper sheet or similar recording medium. One of conventional fixing device uses a heat roller to be heated and a press roller pressed against the heat roller. The heat roller and press roller fix a toner image on a paper sheet with heat and pressure while conveying the paper sheet. A halogen lamp, halogen heater or similar heat source has traditionally been disposed in the heat roller for heating the heat source to a preselected temperature necessary for fixation.
It is a common practice with the above-described fixing device to heat the heat roller to a preselected surface temperature, e.g., 180xc2x0 C. and then maintain it at a temperature that is about 50% to 90% of the above temperature, e.g., 120xc2x0 C. This allows a person to use the image forming apparatus without wasting time. However, even when the fixing device is held in a stand-by state at, e.g., 120xc2x0 C., it naturally consumes much power. From the energy saving standpoint, the fixing device should consume a minimum of energy even in the stand-by state.
In light of the above, there have been proposed various fixing systems featuring a short warm-up time and therefore allowing power supply to be shut off when in a stand-by state. Particularly an induction heating type fixing system is attracting increasing attention because it heats a heat roller, which is formed of conductive metal, by using eddy current derived from an electromagnetic wave.
In an induction heating type fixing device, if the range over which an induction coil is wound differs from a sheet passing width, then so-called hot offset occurs due to excessive temperature elevation in ranges that a paper sheet does not pass. Japanese Patent Laid-Open Publication No. 2000-133627, for example, discloses an induction heating type fixing device capable of obviating hot offset and reducing wasteful power consumption. The fixing device disclosed includes a main coil and an auxiliary coil assigned to a range covering small paper sizes and a range outside of the above range, respectively. The main coil and auxiliary coil are wound round a single bobbin and controlled independently of each other as to current supply. The gap between the coils and a heat roller should preferably be as small as possible in order to efficiently heat the heat roller. To reduce the gap, the end portions of the auxiliary coil are laid inside of the bobbin.
In the configuration taught in the above-mentioned document, the auxiliary coil is positioned at opposite end portions of the bobbin while the main coil is positioned at the center of the same. This brings about a problem that the main coil is laid inside of the bobbin, making it difficult to deal with the end portions of the coils. Further, when any part of the coils snaps, all the coils must be rewound. A period of time necessary for rewinding work is so long, the coils are bodily replaced, i.e., even the coil not snapped is discarded.
On the other hand, the warm-up time of the fixing device can be reduced if the thermal capacity of the heat roller is reduced. For this purpose, the wall thickness of the heat roller may be reduced, as proposed in the past. This, however, brings about another problem that substantially no heat conduction occurs in the heat roller in the axial direction because the heat roller has a core whose thermal conductivity is relatively low. To solve this problem, there has been proposed a system in which a plurality of coils are selectively energized in accordance with sheet size. This allows only necessary part of the heat roller to be heated and thereby further saves energy.
The plurality of coils, however, require a number of leads to be laid between the heat roller and the coils. Therefore, to protect the coils from damage ascribable to, e.g., short-circuiting, a sufficient gap is necessary between the heat roller and the coils and leads. Such a gap increases the distance between the coils and the heat roller and thereby aggravates thermal conversion efficiency. Moreover, a number of leads increase the number of wiring steps and therefore production cost.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 10-10901, 10-20704, 10-153918, 10-282826, 11-316509, 2000-105518, and 2000-56598.
It is a first object of the present invention to provide an induction heating type fixing device capable of accurately controlling the surface temperature of a heat roller for thereby reducing hot offset, an induction heating coil therefor, and a method of producing the coil.
It is a second object of the present invention to provide a low cost, induction heating type fixing device capable of promoting stable heating without lowering thermal conversion efficiency, an induction heating coil therefor, and a method of producing the coil.
It is a third object of the present invention to provide an induction heating type fixing device easy to produce and repair, an induction heating coil therefor, and a method of producing the coil.
It is a fourth object of the present invention to provide an image forming apparatus including the induction heating type fixing device stated above.
In accordance with the present invention, in an induction heating coil including a bobbin formed of a heat-resistant material and a conductor wound round the bobbin, a particular conductor is wound round each of a plurality of bobbins having different diameters and assembled in a telescopic manner.
Also, in accordance with the present invention, an induction heating type fixing device for fixing a toner image on a recording medium, the induction heating type device includes two rollers for conveying the recording medium while nipping it, and an induction heating coil associated with at least one of the rollers for generating an induction magnetic flux. The induction heating coil has a plurality of bobbins formed of a heat-resistant material and each having a particular diameter. The bobbins are assembled in a telescopic manner. A plurality of conductors each are wound round one of the bobbins.
Further, in accordance with the present invention, in an image forming apparatus including an induction heating type fixing device for fixing a toner image on a recording medium, the induction heating type fixing device includes two rollers for conveying the recording medium while nipping it, and an induction heating coil associated with at least one of the two rollers for generating an induction magnetic flux. The induction heating coil has a plurality of bobbins formed of a heat-resistant material and each having a particular diameter. The bobbins are assembled in a telescopic manner. A plurality of conductors each are wound round one of the bobbins.
Moreover, in accordance with the present invention, in a method of producing an induction heating coil for an induction heating type fixing device and including a bobbin for supporting conductors, the bobbin is implemented as a plurality of bobbin members removably connected to each other. After a particular conductor has been wound on each bobbin member, the bobbin members are assembled.